With the maturation of the computer and surrounding technologies, vast amounts of complex, mixed traffic types are transmitted through synchronous optical networks (SONETs). One SONET standard is described in the American National Standards Institute (ANSI) standards T1.105 and T1.106 and in the Bellcore Technical Recommendations TR-TSY-000253. While the SONET standard is popular within the North American continent, the Synchronous Digital Hierarchy (SDH) network, as defined in the ITU-T GR707 document, is prevalent in other regions of the world. The SDH global standards were derived from SONET and include most of the features specified by SONET.
The basic building block of SDH networks is the SDH ring connection. FIG. 1 illustrates a basic SDH ring connection. SDH switch 100 and SDH switch 150 receive optical signals from various devices (not shown in FIG. 1). SDH switch 100 and SDH switch 150 can be coupled to other SDH switches, or other devices that communicate data using optical signals.
SDH switch 100 and SDH switch 150 communicate using two sets of uni-directional signaling pairs. In general, half of the traffic between switches travels over one of the signaling pairs and the other half of the traffic travels over the other signaling pair. SDH switches communicate according to a predetermined protocol, and at a predetermined bit rate.
Legacy telecommunication networks are based on a ‘Plesiochronous Digital Hierarchy’ (PDH) of signals which are not directly compatible between communications protocols used in different regions of the world. The SDH protocol has been developed to act as a common synchronous transport mechanism to transport different kinds of legacy PDH signals. Each of these PDH signals is mapped into Virtual Containers (VCs) and multiple VCs are mapped into SDH frames before being transported. The VCs constitute the payload for the SDH transport system. In the table that follows, bit rates are set forth as bits per second (bps) and multiples thereof, for the common PDH signals. The related virtual container is also listed.
TABLE 1Telco PDH HierarchySignalBit RateChannelsVirtual ContainerDS064kbps1DS0DS11.544Mbps24DS0sVC11E12.048Mbps32DS0sVC12DS26.312Mbps96DS0sVC2E334.368Mbps16E1sVC3DS344.736Mbps28DS1sVC3E4139.264Mbps4E3sVC4
Multiple VC11s, VC12s and VC2s can be mapped into a single VC3 and three VC3s can be mapped into a single VC4. Multiple VC3s (North American standard) or VC4s (Europe and Asia) can be mapped into SDH frames (known as Synchronous Transport Modules or STMs) before being converted into Optical Signals (OC) and sent through fiber. VCs that are mapped into other (higher rate) VCs are known as Tributary Units (TUs). VCs that are mapped into STMs are known as Administrative Units (AUs). The common STMs associated with SDH signals, their bit rates and their transport capacity are shown in the following table.
TABLE 2SDH HierarchySignalBit RateCapacitySTM-0, OC-151.840Mbps1VC3STM-1, OC-3155.520Mbps1VC4STM-4, OC-12622.080Mbps4VC4sSTM-16S-48, OC-482488.320Mbps16VC4sSTM-64, OC-1929953.280Mbps64VC4s
Because of the difference in the STM structure within North American SDH networks and SDH networks in other parts of the world, it is required to re-map SDH payloads to the destination STM structure while crossing network domains. This function is usually implemented in dedicated systems (known as gateways) which extract the SDH payloads from the originating network and re-map them to the STM structure of the target network, before transmitting them.